Vehicle fuel cell power plant warm-up control

ABSTRACT

A fuel cell power plant comprises plural fuel cell stacks ( 10 A- 10 B) capable of generating power independently, and a warm-up circuit ( 33 ) which can independently warm up the fuel cell stacks ( 10 A- 10 C). A controller ( 14 ) estimates a required output power of the power plant within a predetermined time from when the vehicle starts running (S 121 -S 127 ), and determines the number of fuel cell stacks satisfying the required output power (S 106 ). The controller ( 14 ) economizes energy required for warm-up of the fuel cell stacks while providing the required output power by warming up only the determined number of fuel cell stacks before the vehicle starts running.

FIELD OF THE INVENTION

[0001] This invention relates to warm-up control of a fuel cell powerplant for vehicles provided with two or more fuel cell stacks.

BACKGROUND OF THE INVENTION

[0002] Tokkai 2000-173638 published by the Japanese Patent Office in2000 discloses a warm-up device for increasing warm-up efficiency when avehicle fuel cell power plant is started. In this prior art technology,the target power plant has two fuel cell stacks. According to thistechnique, heated water is first circulated only to one of the fuel cellstacks when the power plant is started. After warm-up of this fuel cellstack is completed, heated water is circulated to the other fuel cellstack.

[0003] The driving force required by the vehicle changes with therunning conditions or loading conditions. In a fuel cell power plant forvehicles provided with two or more fuel cell stacks, it is preferredfrom energy efficiency considerations to operate only some fuel cellstacks when the drive force required by the vehicle is small, and tooperate all fuel cell stacks when the driving force required by thevehicle increases.

SUMMARY OF THE INVENTION

[0004] Although the prior art technology is desirable from the viewpointof performing warm-up of the whole power plant efficiently, all the fuelstacks are warmed up in turn, so a given fuel cell stack which does notneed to be operated for the time being may also be warmed up, and thismay consume excessive energy. Moreover, the energy which a fuel cellstack produces will be consumed for warm-up of other fuel cell stacks,and the energy for running may be insufficient.

[0005] It is therefore an object of this invention to estimate arequired output power of a fuel cell power plant for vehicles, and torealize operation of the fuel cell power plant according to the requiredoutput power.

[0006] In order to achieve the above object, this invention provides awarm-up controller for a plurality of fuel cell stacks provided in afuel cell power plant mounted on a vehicle. The fuel cell stacks areindependently operable and generate power for driving the vehicle.

[0007] The controller comprises a warm-up circuit which canindependently warm up the fuel cell stacks, a sensor which detects avehicle running condition, and a programmable controller programmed toestimate a required output power required of the power plant within apredetermined time after the vehicle starts running based on the runningcondition, determine the number of fuel cell stacks to be warmed-upaccording to the required output power, and control the warm-up circuitto warm-up only the determined number of fuel cell stacks.

[0008] This invention also provides a method for controlling warm-up ofa plurality of fuel cell stacks provided in a fuel cell power plantmounted on a vehicle, wherein power plant comprises a warm-up circuitwhich can independently warm up the fuel cell stacks, and the fuel cellstacks are independently operable and generate power for driving thevehicle.

[0009] The method comprises detecting a vehicle running condition,estimating a required output power required of the power plant within apredetermined time after the vehicle starts running based on the runningcondition, determining the number of fuel cell stacks to be warmed-upaccording to the required output power, and controlling the warm-upcircuit to warm-up only the determined number of fuel cell stacks.

[0010] The details as well as other features and advantages of thisinvention are set forth in the remainder of the specification and areshown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic diagram describing the water recirculationstate of a fuel cell power plant for vehicles to which this invention isapplied.

[0012]FIG. 2 is similar to FIG. 1, but showing a different waterrecirculation state.

[0013]FIG. 3 is similar to FIG. 1, but showing another, different waterrecirculation state.

[0014]FIG. 4 is similar to FIG. 1, but showing yet another, differentwater recirculation state.

[0015]FIG. 5 is a flow chart describing a power plant warm-up routineexecuted by a controller according to this invention.

[0016]FIG. 6 is a diagram describing the characteristics of a mapspecifying a required output power and the number of fuel cell stackswhich should be operated, stored by the controller.

[0017]FIG. 7 is a flowchart describing a routine for calculating therequired output power executed by the controller.

[0018]FIG. 8 is similar to FIG. 5, but showing a second embodiment ofthis invention.

[0019]FIG. 9 is similar to FIG. 5, but showing a third embodiment ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Referring to FIGS. 1-4 of the drawings, a fuel cell power plantfor vehicles to which this invention is applied, is provided with apower generation unit 10 comprising three fuel cell stacks 10A-10C. Thefuel cell stacks 10A-10C are connected in parallel to a load L.

[0021] The fuel cell power plant for vehicles is provided with acirculation passage 33 which allows cooling water or warm water tocirculate through the fuel cell stacks 10A-10C in order to maintain thetemperature of the fuel cell stacks 10A-10C within a range required forpower generation, as shown in the figure. A pump 13, a heater 11 whichheats the recirculated water and a radiator 12 in which the temperatureof the recirculated water is reduced, are provided in the circulationpassage 33.

[0022] Three-way valves 21-28 are also provided for switching flowpaths.

[0023] The change-over operation of the three-way valves 21-28,operation of the pump 13 and energization of the heater 11 arecontrolled by a controller 14 according to the state of the power plant.The voltage supplied to the load L from the power generation unit 10 isdetected by a voltmeter 31, the current amount is detected by an ammeter32, and detected data are input to the controller 14 as signals,respectively. Here, the load L is an electric motor which drives thevehicle. The vehicle speed V is also input to the controller 14 as asignal from a vehicle speed sensor 34 which detects the running speed ofthe vehicle.

[0024] The controller 14 comprises a microcomputer provided with acentral processing unit (CPU), read-only memory (ROM), random accessmemory (RAM), and input/output interface. The controller may alsocomprise plural microcomputers. A display/input unit 40 which informs adriver of the vehicle about the running state of the vehicle and intowhich the driver can input commands, is connected to the controller 14.

[0025] To raise the temperature of the fuel cell stack 10A (10B, 10C) towithin a range suitable for power generation when the power plantstarts, the controller 14 circulates warm water through the circulationpassage 33 prior to start of power generation by applying one of thepatterns shown in FIGS. 1-3.

[0026] Referring firstly to FIG. 1, when only the fuel cell stack 10A iswarmed up, the controller 14 operates the three-way valves 21, 22 and24-27 so that water flows along the path shown by the arrows in thefigure, energizes the heater 11 and operates the pump 13. The warm waterheated by the heater 11 flows through the circulation passage 33 alongthe path shown by the arrows in the figure while warming the fuel cellstack 10A. To prevent the temperature of the warm water from falling,the three-way valves 24, 26 are operated so that the warm water bypassesthe radiator 12.

[0027] Next, referring to FIG. 2, when the fuel cell stacks 10A, 10B arewarmed up, the controller 14 operates the three-way valves 21-28 so thatwater flows along the path shown by the arrows in the figure, energizesthe heater 11 and operates the pump 13. The warm water heated by theheater 11 flows through the circulation passage 33 along the path shownby the arrows in the figure, and warms the fuel cell stacks 10A, 10B.Also in this case, to prevent the temperature of the warm water fromfalling, the three-way valves 24, 26 are operated so that the warm waterbypasses the radiator 12.

[0028] Next, referring to FIG. 3, when all three of the fuel cell stacks10A-10C are warmed up, the controller 14 operates the three-way valves21-28 so that water flows along the path shown by the arrows in thefigure, energizes the heater 11 and operates the pump 13. The warm waterheated by the heater 11 flows through the circulation passage 33 alongthe path shown by the arrows in the figure, and warms the fuel cellstacks 10A-10C. Also in this case, to prevent the temperature of thewarm water from falling, the three-way valves 24, 26 are operated sothat the warm water bypasses the radiator 12.

[0029] After warm-up of only the fuel cell stack 10A along the pathshown in FIG. 1, or after warm-up of only the fuel cell stacks 10A, 10Balong the path shown in FIG. 2 is completed, heat will be generatedaccompanying power generation performed by these fuel cell stacks.

[0030] The controller 14 makes water flow through all of fuel cellstacks 10A-10C according to the pattern shown by the solid line arrowsin FIG. 4 using this surplus heat so as to warm up the fuel cell stack10C or the fuel cell stacks 10B, 10C which were not previously warmedup. That is, the controller 14 stops energizing the heater 11 andoperates the three-way valves 21, 25 so as to cause the water dischargedfrom the pump 13 to flow into the fuel cell stack 10A without passingthrough the heater 11.

[0031] Consequently, the recirculated water absorbs heat from the fuelcell stack 10A or the fuel cell stacks 10A, 10B which started powergeneration, raises the temperature, and warms the fuel cell stack 10C orthe fuel cell stacks 10B, 10C which were not warmed up.

[0032] Further, after warm-up of all the fuel cell stacks 10A-10C iscompleted, the controller 14 operates the three-way valves 24-26, andmakes the water discharged from the pump 13 to flow into the fuel cellstack 10A from the three-way valve 26 via the radiator 12, three-wayvalve 24, three-way valve 25 and three-way valve 21 as shown by thedashed line arrows in the figure. In this water recirculation pattern,the water recirculated in the recirculation passage 33 absorbs heat fromthe fuel cell stacks 10A-10C under power generation, and the heat isradiated by the radiator 12.

[0033] Even when warm-up of all the fuel stacks 10A-10C is notcompleted, if the temperature of the recirculated water risesconsiderably, the recirculated water is likewise cooled by the radiator12. In this case, the water recirculation pattern shown by the dashedline arrows is applied instead of the water recirculation pattern shownby the solid line arrows in the figure.

[0034] Now, when the power plant is started, the controller 14 selectsthe water circulation patterns of FIGS. 1-3 by executing a power plantwarm-up routine shown in FIG. 5 before the power plant starts generatingpower, and warms up the vehicle fuel cell stack required to drive thevehicle under the given conditions.

[0035] This routine is executed once prior to starting power generationwhen the power plant starts.

[0036] Referring to FIG. 5, firstly in a step S101, the controller 14detects that the driver of the vehicle has turned a power plant startpreparation switch ON. The start preparation switch is provided on theabove-mentioned display/input unit 40.

[0037] In a following step S102, a setting mode of the output powerrequired of the power plant is determined. The output power setting modedetermines whether the output power required of the power plant is to bespecified by the driver, or set automatically based on the past runningpattern. This selection is performed by the driver by operating thepower plant start preparation switch. In the step S102, based on thesignal from the power plant start preparation switch, the controller 14determines whether the manual setting mode or automatic setting mode isselected.

[0038] When the automatic setting mode is selected as a result of thedetermination of the step S102, the controller 14 reads the requiredoutput power Ln prestored in the memory in a step S103. The requiredoutput power Ln is the estimated value of the maximum output powerrequired of the power plant within a predetermined time from starting ofthe power plant. Here, the predetermined time is set to ten minutes. Therequired output power Ln is updated by execution of a routine shown inFIG. 7, described below.

[0039] In a following step S105, the estimated required output power Lnis displayed on the display/input unit 40, and the agreement of thedriver is requested. The driver inputs whether or not he/she agrees tothe displayed required output power on the display/input unit 40.

[0040] When the manual setting mode is chosen as a result of thedetermination of the step S102, or when the driver does not agree to therequired output power Ln in the step S105, the driver is asked formanual input of the required output power in a step S108, and thecontroller 14 reads the required output power input to the display/inputunit 40.

[0041] When the driver agrees to the required output power Ln in thestep S105, or after reading the required output power input to thedisplay/input unit 40 in the step S108, the controller 14 performs theprocessing of a step S106. In the step S106, the number of fuel cellstacks which should start is determined by referring to a map having thecharacteristics shown in FIG. 6 which is prestored in the memory of thecontroller 14. According to this map, the number of fuel cell stackswhich should start increases according to the required output power.

[0042] In a following step S107, one of the above patterns of FIGS. 1-3is selected and warm-up of the fuel cell stack(s) is performed accordingto the number of fuel cell stacks that will be operated for satisfyingthe required output power.

[0043] Next, a routine for generating a map of the required output powerLn used in the step S103 of FIG. 5 will be described referring to FIG.7. This routine is executed only once immediately after the vehicle hasstarted running independently of the power plant warm-up routine of FIG.5. The required output power Ln updated by this routine is stored in thememory of the controller 14, and is used in the step S103 of FIG. 5 onthe next occasion when the power plant is operated. The initial value ofthe required output power Ln is set to the average value of the requiredoutput power of a fuel cell power plant for a vehicle of identicalspecification.

[0044] In a step S121, the controller 14 resets a timer value T to zero.

[0045] In a following step S122, it is determined whether or not thevehicle speed V is larger than zero, i.e., whether or not the vehicle isrunning. When the vehicle speed V is not larger than zero, theprocessing of the steps S121, S122 is repeated without proceeding tofurther steps until the vehicle speed V becomes larger than zero.

[0046] When the vehicle speed V becomes larger than zero, the outputpower of the power plant is calculated from the product of the outputvoltage of the power generation unit 10 input from the voltmeter 31 andthe output current of the power generation unit 10 from the ammeter 32in a step S123. The maximum of the output power after starting theroutine is stored in the memory of the controller 14 as the maximumoutput power Lmax. The output power calculated by this execution of theroutine and the maximum output power Lmax stored in the memory arecompared, and when the output power calculated on this occasion islarger than the maximum output power Lmax, the maximum output power Lmaxis updated by the value of the load calculated on this occasion.

[0047] In a following step S124, the timer value Tis compared with apredetermined time. The predetermined time is ten minutes as mentionedabove. Here, the units of the timer value Tare in seconds, and thepredetermined time is 600 seconds.

[0048] When the timer value T does not reach the predetermined time, thecontroller 14 stands by for one second in a step S125, increments thetimer value Tin a step S126, and repeats the processing of the stepsS123, S124.

[0049] When the timer value T exceeds the predetermined time of 600seconds in the step S124, the controller 14 performs weighted averageprocessing of the required output power by the following equation (1) ina step S127.

Ln=r L _(n-1)+(1−r)Lmax  (1)

[0050] where, r=weighting coefficient.

[0051] That is, the required output power L_(n-1) calculated on theimmediately preceding occasion the routine was executed and the maximumload Lmax calculated on this execution of the routine are averaged byapplying a weighting coefficient r, and the result is set as a newrequired output power Ln. The weighting coefficient r is a constantlarger than zero, and smaller than unity.

[0052] After the processing of the step S127, the controller 14terminates the routine.

[0053] According to the power plant warm-up routine of FIG. 5 which thecontroller 14 performs when the power plant starts, as warm-up of thefuel cell stacks is performed by applying one of the warm-up patterns ofFIGS. 1-3 based on the past track record of the required output powerduring ten minutes after the vehicle start, only the number of fuel cellstacks according to the required output power is warmed up first whenthe power plant is started. When warm-up is completed, the power plantstarts power generation using the fuel cell stack which completedwarm-up, and the vehicle starts running using the output power of thepower plant. As for the fuel cell stacks which were not warmed up duringstarting, after the vehicle starts running, warm-up is performedaccording to a water recirculation pattern represented by the solidarrows in FIG. 4 which uses the surplus heat of the fuel cell stacks inoperation. Thus, finally, all the fuel cell stacks 10A-10C will be in astate where they are able to generate power. Thus, the energy consumedby warm-up can be suppressed to the minimum by warming up only the fuelcell stacks required to satisfy the required output power within thepredetermined time, i.e., ten minutes, after the vehicle starts running,when the power plant starts.

[0054] Next, referring to FIG. 8, a second embodiment of this inventionwill be described.

[0055] In this embodiment, the vehicle is provided with a car navigationsystem 41 shown in FIGS. 1-4 which collects information on the roadsituation from the present location to the destination of the vehicle.The controller 14 determines the required output power of the powerplant based on the information from the car navigation system 41.

[0056] For this purpose, the controller 14 performs a power plantwarm-up routine shown in FIG. 8 which replaces the routines of FIG. 5and FIG. 7 of the first embodiment.

[0057] The processing of a step S141 is identical to that of the step101 of the routine of FIG. 5, the processing of a step S142 is identicalto that of the step S102 of the routine of FIG. 5, and the processing ofsteps S146-S149 is identical to that of the steps S105-S108 of theroutine of FIG. 5, respectively.

[0058] When the automatic setting mode is set in the step S142, thecontroller 14 asks the driver to input the destination to the navigationsystem 41 in a step S143. The controller 14 does not proceed to thefollowing step S144 until it confirms input of the destination by thedriver.

[0059] In a following step S144, the controller 14 reads the roadsituation from the present location of the vehicle to the destinationfrom the car navigation system 41. Here, the car navigation system 41may be a system which reads road information from a storage medium suchas a memory or a disk based on the destination input and the presentlocation, or it may be a system which receives road information from aserver of an information center via a communication means with theoutside such as a cellular phone. The road situation which thecontroller 14 reads includes the following information about the roadswhich the driver intends to take within a predetermined time after thevehicle starts running, such as the type of road, i.e., highways orordinary roads, whether the roads are flat, uphill or downhill, andwhether these roads are congested. Here, the predetermined time is tenminutes.

[0060] In a following step S145, the controller 14 estimates therequired output power Ln of the power plant required for the running ofthe vehicle within the predetermined time after the vehicle startsrunning based on the read road situation. Specifically, a running speedis set from the type of the road selected for running taking account ofwhether or not it is congested. Once the running speed is set, therequired power plant output power Ln is then calculated from the runningspeed and the slope of the road to be taken.

[0061] If a map of required output power according to road slope andrunning speed is prepared beforehand experimentally or by simulation andstored in the memory of the controller 14, the required output power Lncan be calculated by searching the map in the step S145. Instead of amap, an approximation which calculates the required output power Ln fromthe road slope and running speed may be used. The calculated requiredoutput power Ln is displayed on the display/input unit 40.

[0062] In a following step S146, as in the step S105 of FIG. 5, theagreement of the driver is requested regarding the displayed requiredoutput power Ln. When the driver does not agree, or when the driverchooses manual input in the step S142, manual input of the requiredoutput power Ln via the display/input unit 40 is requested in a stepS149 as in the step S108, and the controller 14 reads the inputtedrequired output power Ln. Instead of inputting the required output powerLn, the driver may input road conditions into the display/input unit 40,and the controller 14 may determine the required output power Ln fromroad conditions by the same method as that of the step S145.

[0063] In the step S146, the driver may not agree to the required outputpower Ln for example when he/she selects a road other than that set bythe car navigation system 41, or when he/she knows of circumstances ofthe road to be taken other than the road slope and running speed usedfor calculation of the required output power Ln, such as information onroad works.

[0064] When the required output power Ln is determined in the step S146or step S149, in steps S147, S148, the controller 14 performs identicalprocessing to that of the steps S106, S107 of FIG. 5, select one of theabove patterns of FIGS. 1-3 based on the required output power Ln, andperform warm-up of the fuel cell stack(s) 10A (10B, 10C) that will beoperated for satisfying the required output power Ln.

[0065] Next, referring to FIG. 9, a third embodiment of this inventionwill be described.

[0066] In this embodiment, as in the second embodiment, the controller14 estimates the required output power Ln in combination with the carnavigation system 41. For this purpose, a routine shown in FIG. 9 isperformed instead of the routine of FIG. 8.

[0067] The processing of steps S161, S162 of the routine of FIG. 9 isidentical to that of the steps S141, S142 of the routine of FIG. 8.Also, the processing of steps S165-168 is identical to that of the stepsS146-S149 of the routine of FIG. 8.

[0068] When the automatic setting mode is chosen in the step S162, thecontroller 14 collects information on all the roads that the vehicle cantake within the predetermined time from the present location of thevehicle from the car navigation system in a step S163. The kind ofinformation collected is identical to that in the second embodiment.

[0069] In a following step S164, the controller 14 estimates therequired output power for all the roads for which information wascollected. The method of estimating the required output power isidentical to that of the second embodiment. The maximum value Ln of therequired output power is displayed on the display/input unit 40.

[0070] In a following step S165, as in the step S126 of FIG. 8, thecontroller 14 requests the agreement of the driver regarding thedisplayed required output power Ln. Here, the driver may not agree tothe required output power Ln for example when the driver knows thesituation of the road that he/she intends to take beforehand, and therequired output power deemed necessary is different from the displayedvalue.

[0071] When a driver does not agree to the required output power Lndisplayed in the step S165, or when the automatic mode is not chosen inthe step S162, in a step S168, as in the step S149 of FIG. 8, manualinput of the required output power Ln to the display/input unit 40 isrequested, and the controller 14 reads the inputted required outputpower Ln.

[0072] After determining the required output power Ln in the step S165or step S168, in steps S166, S167, the controller 14 performs identicalprocessing to that of the steps S106, S107 of FIG. 5, selects one of thepatterns of FIGS. 1-3 based on the required output power Ln, andperforms warm-up of the fuel cell stack(s) 10A (10B, 10C) that will beoperated for satisfying the required output power Ln.

[0073] In this embodiment, although the controller 14 collectsinformation on all the roads that can be taken within the predeterminedtime, information may be more simply collected about the situation ofall the roads within a predetermined distance from the present vehiclelocation. The predetermined distance may be set to, for example, fivekilometers.

[0074] In all the above embodiments, the number of fuel cell stackswhich are warmed up is determined according to the road situation sothat the output power required Ln for running the vehicle within thepredetermined time from when the vehicle starts running can be obtained,and the energy required for warm-up can be suppressed to the minimumwithout causing a shortage of output power.

[0075] Execution of each of the routines of FIGS. 5, 8, 9 starts whenthe power plant start preparation switch is turned ON. After warm-up ofthe fuel cell stack(s) 10A (10B, 10C) is completed by execution of theroutines, processing may be continued by one of the following methods.

[0076] In one method, warm-up completion of the fuel cell stacks may bedisplayed on the display/input unit 40, and power generation by thepower plant started by turning a main switch ON. The main switch may beprovided in the display/input unit 40, or provided independently of thedisplay/input unit 40.

[0077] Another method is that the power plant starts power generationautomatically when warm-up of the fuel cell stack(s) 10A (10B, 10C)according to the required output power Ln is completed.

[0078] In all the above embodiments, the predetermined time is set toten minutes. This is based on the time taken to complete warm-up ofother fuel cell stacks by the surplus heat of the fuel cell stack(s)first operated after the vehicle starts running, i.e., on the timerequired until warm-up of all the fuel cell stacks 10A-10C is completedunder the recirculation pattern shown by the solid line arrows in FIG. 4from when the vehicle starts running. Therefore, it is preferred todetermine the predetermined time by experiment or simulation.

[0079] The contents of Tokugan 2001-385713, with a filing date of Dec.19, 2001 in Japan, are hereby incorporated by reference.

[0080] Although the invention has been described above by reference tocertain embodiments of the invention, the invention is not limited tothe embodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings.

[0081] For example, in the above embodiments, this invention is appliedto a power plant comprising the three fuel cell stacks 10A-10C, but itmay be applied to all power plants provided with two or more fuel cellstacks which can be individually warmed up.

INDUSTRIAL FIELD OF APPLICATION

[0082] As mentioned above, this invention estimates the output powerrequired of a power plant within a predetermined time from when thevehicle starts running, and warms up only the number of fuel cell stacksaccording to the required output power prior to start of running.Therefore, the energy consumed by warm-up can be reduced in a fuel cellpower plant for vehicles wherein energy cannot be supplied from outside.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A warm-up controller fora plurality of fuel cell stacks (10A-10C) provided in a fuel cell powerplant mounted on a vehicle, the fuel cell stacks (10A-10C) beingindependently operable and generating power for driving the vehicle, thecontroller comprising: a warm-up circuit (11, 13, 21-28) which canindependently warm up the fuel cell stacks (11A-10C); a sensor (31, 32,40, 41) which detects a vehicle running condition; and a programmablecontroller (14) programmed to: estimate a required output power requiredof the power plant within a predetermined time after the vehicle startsrunning based on the running condition (S103, S108, S121-S127, S145,S149, S164, S168); determine the number of fuel cell stacks (10A-10C) tobe warmed-up according to the required output power (S106, S147, S166);and control the warm-up circuit (11, 13, 21-28) to warm-up only thedetermined number of fuel cell stacks (10A-10C) (S107, S148, S167). 2.The warm-up controller as defined in claim 1, wherein the controller(14) is further programmed to control the warm-up circuit (11, 13,21-28) to warm-up only the determined number of fuel cell stacks(10A-10C) before the vehicle starts running.
 3. The warm-up controlleras defined in claim 1 or claim 2, wherein the sensor (31, 32, 40, 41)further comprises a sensor (31, 32) which detects the output powerrequired of the power plant, and the controller (14) is furtherprogrammed to monitor the output power required of the power plant forthe predetermined time after the vehicle starts running (S122-S126), andestimate the required output power which is to be applied to determinethe number of fuel cell stacks (10A-10C) for warm-up on a next occasionbased on a maximum value of the required output power monitored in thepredetermined time (S127).
 4. The warm-up controller as defined in claim1 or claim 2, wherein the sensor (31, 32, 40, 41 comprises a navigationsystem (41) which detects a condition of a road leading from a currentlocation of the vehicle to a destination of the vehicle, and thecontroller (14) is further programmed to estimate the required outputpower based on the condition of the road leading from the currentlocation to the destination (S145).
 5. The warm-up controller as definedin claim 4, wherein the road condition comprises a slope of a road and aspeed with which a vehicle can travel on the road.
 6. The warm-upcontroller as defined in claim 1 or claim 2, wherein the sensor (31, 32,40, 41) comprises a navigation system (41) which detects a condition ofall roads leading from a current location of the vehicle to adestination of the vehicle, and the controller (14) is furtherprogrammed to estimate the required output power based on the roadcondition of all the roads leading from the current location to thedestination (S167).
 7. The warm-up controller as defined in claim 6,wherein the road condition comprises a slope of a road and a speed withwhich a vehicle can travel on the road.
 8. The warm-up controller asdefined in claim 1 or claim 2, wherein the sensor (31, 32, 40, 41)comprises a running condition input device (40), and the controller (14)is further programmed to estimate the output power required of the powerplant within the predetermined time from when the vehicle starts runningbased on a running condition input by a driver of the vehicle to theinput device (40) (S108, S149, S168).
 9. A warm-up controller for aplurality of fuel cell stacks (10A-10C) provided in a fuel cell powerplant mounted on a vehicle, the fuel cell stacks (10A-10C) beingindependently operable and generating power for driving the vehicle, thecontroller comprising: a warm-up circuit (11, 13, 21-28) which canindependently warm up the fuel cell stacks (10A-10C); means (31, 32, 40,41) for detecting a vehicle running condition; means (14, S103, S108,S121-S127, S145, S149, S164, S168) for estimating a required outputpower required of the power plant within a predetermined time after thevehicle starts running based on the running condition; means (14, S106,S147, S166) for determining the number of fuel cell stacks (10A-10C) tobe warmed-up according to the required output power; and means (14,S107, S148, S167) for controlling the warm-up circuit (11, 13, 21-28) towarm-up only the determined number of fuel cell stacks (10A-10C).
 10. Amethod for controlling warm-up of a plurality of fuel cell stacks(10A-10C) provided in a fuel cell power plant mounted on a vehicle, thepower plant comprising a warm-up circuit (11, 13, 21-28) which canindependently warm up the fuel cell stacks (10A-10C), and the fuel cellstacks (10A-10C) being independently operable and generating power fordriving the vehicle, the method comprising: detecting a vehicle runningcondition (31, 32, 40, 41); estimating a required output power requiredof the power plant within a predetermined time after the vehicle startsrunning based on the running condition (S103, S108, S121-S127, S145,S149, S164, S168); determining the number of fuel cell stacks (10A-10C)to be warmed-up according to the required output power (S106, S147,S166); and controlling the warm-up circuit (11, 13, 21-28) to warm-uponly the determined number of fuel cell stacks (10A-10C) (S107, S148,S167).